WO2014111264A1

WO2014111264A1 - COATING METHOD FOR PRODUCING (Al,Cr)2O3-BASED COATINGS WITH ENHANCED PROPERTIES

Info

Publication number: WO2014111264A1
Application number: PCT/EP2014/000119
Authority: WO
Inventors: Christian Martin Koller; Richard RACHBAUER; Peter Polcik; Jürgen RAMM; Joerg PAULITSCH; Paul Heinz Mayrhofer
Original assignee: Oerlikon Trading Ag, Trübbach
Priority date: 2013-01-18
Filing date: 2014-01-17
Publication date: 2014-07-24
Also published as: EP2954091A1; EP2954091B1

Abstract

Coating method for coating a substrate with at least one aluminium oxide comprising coating layer, the layer being produced by physical vapour deposition from at least one metallic target comprising aluminium as main component in a coating chamber containing oxygen as reactive gas, characterized in that the target further comprises chromium as second main component and boron or iron as minor component or boron and iron as minor components, or the target further comprises iron as minor component. The target is preferably made by powder metallurgy and the substrate temperature during coating process is not higher than 600 °C.

Description

Coating method for producing (AI,Cr)₂0₃-based coatings with enhanced properties

The present invention relates to a coating process for producing aluminium chromium oxide coatings by using targets comprising aluminium and chromium as main component and additionally boron and/or iron as minor component. In terms of the present description aluminium chromium oxide coatings are referred to (AI,Cr)₂0₃, if they are produced without additional dopants, or referred to (AI,Cr)₂0₃-based, if they comprise additional dopants, more particularly referred to AI-Cr-B-0 or Al- Cr-Fe-0 if they are produced with B or Fe as dopant.

State of the art

(AI,Cr)₂0₃ coatings exhibiting corundum structure (ct-phase) are known for their outstanding mechanical, tribological and thermal properties. However, stabilising the desired corundum structure during deposition by using low temperature physical vapour processes, like for example cathodic arc evaporation, turned out to be challenging, particularly if Al/Cr metallic ratios of 2 or more in the oxide coatings want to be attained. Furthermore, recent studies on depositing (Alo.7Cro.₃)₂0₃ showed that due to the reactive process in oxygen atmosphere aluminium rich oxide islands are formed at the target surface, influencing the arcing behaviour as well as the resulting film quality. This problem can be solved, according to the state of the art, by using Al-Cr targets with more than 80 at.-% Al content. However, it is known that an Al/Cr ratio of 4 or more in the (AI,Cr)₂0₃ or (AI,Cr)₂0₃-based oxide coating prevents the formation of (AI,Cr)₂0₃ stable corundum structures in the coating.

Objective of the present invention

The main objective of the present invention is to provide a coating method for producing (AI,Cr)₂0₃ coatings or (AI,Cr)₂0₃-based coatings exhibiting the desired (AI,Cr)₂0₃ corundum structure, the Al/Cr ratio in the coatings being preferably above 2, whereby the coating process is conducted at low process temperatures and accordingly at substrate temperatures preferably not higher than 600 °C.

Description of the present invention

Using targets comprising aluminium (Al) and chromium (Cr) as main components and additionally boron (B) and/or iron (Fe) as minor component it was possible to produce (AI,Cr)₂0₃ coatings or (AI,Cr)₂0₃-based coatings at low substrates temperatures of maximal 600 °C, which exhibit an increased amount of corundum structure in comparison with coatings deposited from targets which don't contain B and/or Fe as minor component according to X-ray examinations.

For attaining a better understanding of the invention, some examples of coating methods according to the present invention will be described in the following. The present invention is however not limited to the particular embodiments described below as examples of the present invention.

Example 1:

AlCr-targets made by powder metallurgy and containing small amounts of B and/or Fe were used as source material for the synthesis of aluminium chromium oxide coatings in a coating chamber containing oxygen as a reactive gas. Different aluminium chromium oxide based coatings were formed by means of cathodic arc evaporation from the AI-Cr-B-targets as well as from Al-Cr-Fe- targets. Surprisingly, formation of oxide islands which is usually observed at the surface of AlCr- targets having an element composition AI:Cr 70:30 in atomic percentage by producing aluminium chromium oxide coatings at high oxygen flows (e.g. using oxygen flows of about 1000 seem), doesn't occur in the same grade at the surface of the used AI-Cr-B-targets. Furthermore, it was possible to stabilize aluminium chromium oxide coatings in its solid solution a structure by using Al-Cr-Fe- or Al- Cr-B-Fe-targets.

For this reason, the inventors investigated the effects of using powder metallurgical AICrFe- and Al- Cr-B-targetsduring deposition of aluminium chromium oxide based coatings by means of cathodic arc evaporation in oxygen atmosphere in more detail, the used targets having B and Fe contents ranging from 1 to 5 at%.

Increased B content was related to significant reduction of oxide formation on the surface of the AI- Cr-B-targets and a densification in the film morphology of the produced aluminium chromium oxide coatings. However, increased B contents resulted in a change in the structure detected by X-ray examinations. In contrast, the target oxide island formation could not be significantly influenced by using the AI-Cr-Fe-targets having Fe content ranging from 1 to 5 at%. Nevertheless, analysis of X-ray and transmission electron microscopy examinations indicated that even low amounts of Fe in the Al- Cr-Fe-targets can reduce the formation of cubic crystalline structure in the produced aluminium chromium oxide coatings, allowing in this way the production of more thermally stable a alumina structure.

For obtaining the results mentioned above several coating processes were carried out. In some cases it was used as process gas only oxygen and in other cases a mix gas containing oxygen and argon. Oxygen flows of for example 300 and 1000 seem were used for the experiments and in each case the total pressure in the coating chamber was maintained constant (the total pressure value for these experiments was for example in some cases about 1.1 Pa). Bias voltage and arc current at the cathode were also maintained constant during coating process.

Examples 2 and 3: 2l

AlCr-targets made by powder metallurgy and containing small amounts of B or Fe were used as cathodes for the evaporation by the cathodic arc and for the synthesis of aluminium chromium oxide coatings in a coating Chamber containing oxygen as reactive gas. Different aluminium chromium oxide based coatings were formed by means of cathodic arc evaporation from the AI-Cr-B-targets as well as from AI-Cr-Fe-targets. The chemical compositions of the utilized targets and the synthesized coatings are summarized in Table 1. To achieve the best measurement accuracy, the composition measurements of the AI-Cr-0 and AI-Cr-Fe-0 coatings were performed by energy dispersive X-ray diffraction (EDX) and the compositional analysis of the AI-Cr-B-0 coatings was performed by elastic recoil detection analysis (E DA).

Usually, the formation of oxide islands is observed at the surface of Al-Cr-targets having an elemental composition of Al/Cr 70 at.% / 30 at.% utilizing high oxygen flows (e.g. using oxygen flows of about 1000 seem). During synthesis of AI-Cr-B-0 coatings from Al-Cr-B targets (Cr was replaced by 1, 2 or 5 at.% B in the target, respectively) at an oxygen flow of 1000 seem the formation of oxide islands on the target surface was surprisingly not observed anymore. This is demonstrated by the SEM micrographs in Figure 1, which shows the target surface after operation at a process temperature of 550°C for 90 min for (a) Al-Cr 70/30 targets, as well as for Al-Cr-B targets with 70/29/1, 70/28/2, 70/25/5 in (b), (c), and (d), respectively. Figure 2 shows the X-ray diffractograms for the corresponding Al-Cr-B-0 coatings which were synthesized from these targets. No improvement or stabilization of the corundum structure could be detected for all of these coatings in comparison to the coating obtained from the un-doped target. This means that B doping effectively helps to avoid the formation of oxide islands, however, does not support the corundum structure in the coating with increasing B content according to the X-ray measurements. Yet another influence of the B doping could be detected on the morphology of the AI-Cr-B-0 coatings. The morphology became denser with increasing B content in the AI-Cr-B-0 coatings, which is reflected in Figure 3 (a-d), where scanning electron microscopy (SEM) fracture cross sections of the synthesized coatings from the Al-Cr-B targets with 70/30/0, 70/29/1, 70/28/2, and 70/25/5, respectively, are presented. This refinement of the coating morphology goes along with reduced droplet density and thus a smoother surface of the AI-Cr-B-0 coatings with increasing B content, compared to the un-doped AI-Cr-0 coatings, see Figure 4 (a-d) for comparison.

il

Particularly, using targets comprising aluminium (Al) and chromium (Cr) as main components and additionally minor fractions of Fe, it was possible to produce AI-Cr-Fe-0 coatings exhibiting increased Al/Cr ratio and a strongly stabilized corundum structure. The coating processes were conducted at substrate temperatures of 600 °C and below. Surprisingly, in this way, it was possible to replace substantially and in some cases even completely Cr with Fe in the (AI,Cr)₂0₃ structure and still anyhow to achieve formation of corundum structures in the coating for high Al contents, inclusive for Al contents in the coating up to 90 at.%.

In a series of experiments using powder metallurgical Al-Cr-Fe targets having Fe additions of 1, 2 and 5 at.%, where Fe replaces Al as well as Cr. The obtained results are shown in Table 1.

In contrast to B, the target oxide island formation could not be significantly influenced using Al-Cr-Fe- targets with Fe content ranging from 1 to 5 at%. This is demonstrated in Figure 5, where segments of the Al-Cr-Fe targets with 70/30/0 (a), 69.5/29.5/1 (b), 69/29/2 (c), and 67.5/27.5/5 (d) at.% are shown after operation in 1000 seem oxygen flow at 550°C for 90 min. Nevertheless, X-ray analysis and transmission electron microscopy examinations indicated that even low amounts of Fe in the Al- Cr-Fe-targets can increase the peak intensity of the corundum structure drastically and reduce the formation of cubic crystalline structure in the produced aluminium chromium oxide coatings, allowing in this way the formation of the corundum structure. This trend can be seen in the XRD pattern in Figure 6, where the peak intensity of the peaks at e.g 34.5 and 37.0 increases with increasing amount of Fe in the coatings. The addition of Fe replacing especially Cr in the Al-Cr targets utilized for the oxide synthesis shows two possible approaches to generate or stabilize the corundum structure in (AI,Cr)₂0₃ coatings. The Al/Cr ratio can be increased in the targets from the current ratio of about 2 to 9 and even higher and it is still possible to achieve the corundum structure in the coatings, if Fe replaces the Cr. In addition, pure Al targets, i.e. without Cr, but with Fe additions also results in coatings which show corundum structure for Fe concentrations between 5 and 20 at.%. This means that Fe acts as a more efficient "catalyser" for the formation of the corundum structure in the synthesized oxide coating than Cr. This was proved for Al/Fe targets with an atomic concentration of 84 at.% Al and 16 at.% Fe for which coatings with corundum structure were obtained, but no corundum structure could be observed if the same Cr concentration was utilized in the target.

Tables and Figures:

Table 1: List of exemplary target compositions and corresponding coating compositions, measured by EDX and ERDA. The composition of the AI-Cr-0 and AI-Cr-Fe-0 coatings could be measured by EDX, whereas for determination of the B-content ERDA measurements were performed.

Figure 1: Cathode surface images of AI-Cr-B-cathodes after operation for 90 min at 550°C in oxygen atmosphere (flow of 1000 seem 02). From (a) to (d) the B-content in the target increases from 0 at.% to 5 at.% at the expense of Cr, which results in an effective reduction of oxide island formation, as to be seen in the magnified subsegments of the targets.

Figure 2: X-ray pattern of AI-Cr-B-0 coatings, deposited in 1000 seem oxygen flow, at 550°C for

90 min. SEM -cross sections of AI-Cr-B-0 coatings, deposited in 1000 seem oxygen flow, at 550°C for 90 min. Oensification of coating structure, as well as lower amount of droplets, is visible with increased B-content from (a) to (d) with increasing B content in the coating.

SEM surface images of of A!-Cr-B-0 coatings, deposited in 1000 seem oxygen flow, at 550% for 90 min. Increasing B content in the coatings from (a) to (d), results in a reduction of droplets in the coatings and thus smoother surface topographies.

Figure 5: Cathode surface images of Al-Cr-Fe cathodes after operation for 90 min at 550T in oxygen atmosphere (flow of 1000 seem 02). From left to right, (a) to (d) the Fe- content in the target increases from 0 to 5 at. , respectively, which has no effect on the formation of oxide islands.

Figure 6: X-ray pattern of Ai-Cr-Fe-O coatings, deposited in 1000 seem oxygen flow, at 550°C for 90 min, where a significant increase of the corundum type structure in the coatings can be observed with increasing Fe content in the coating. The indicated Fe contents refer to the target compositions.

Figure 7: Shows table 1

Concise description of the invention:

Concretely, the present invention relates to a costing method for coating a substrate with at least one aluminium oxide comprising coating layer, the layer being produced by physical vapour deposition from at least one target, preferably a metallic target, comprising aluminium as main component in a coating chamber containing Oxygen as reactive gas, characterized in that:

- the target further comprises chromium as second main component and boron as minor component or iron as minor component or boron and iron as minor components.

Or

the target further comprises iron as minor component. Preferably, the target is made by powder metallurgy.

According to a preferred embodiment of the present invention, the physical vapour deposition process is a sputtering process, or a magnetron sputtering process, or a high power impulse magnetron sputtering process, or a cathodic arc evaporation process.

The substrate temperature during coating process should not be higher than 600 °C, preferably it should not be higher than 550 °C.

Preferably, during the physical vapour deposition a bias voltage is applied to the substrate that is being coated.

Preferably, the oxygen contained in the coating chamber is introduced by using a total oxygen flow of at least 300 seem, in some cases more preferably of at least 1000 seem.

Also in some cases using an oxygen partial pressure in the coating chamber of about 1.1 Pa could be advantageously.

If the target comprises aluminium and chromium, the target can be used for producing an aluminium chromium oxide based coating layer, In this case an atomic ratio Al/Cr higher than 2 in the target can be advantageously. A ratio Al/Cr of approximately 70/30, or higher than 70/30, can be particularly advantageously for some applications, such as for coating cutting tools.

If the target comprises aluminium and iron only, the target can be used for producing an aluminium iron oxide coating layer and the atomic ratio Al/Fe in the target is at least 70/30, preferably Al/Fe > 70/30. If the target comprises boron, the boron content in atomic percentage in the target should not be higher than 20 at.-%. Boron contents in the target between 1 at.-% and 5 at.-% can be particularly advantageously for some applications.

If the target comprises iron, the iron content in atomic percentage in the target should not be higher than 20 at.-%. Iron contents in the target between 1 at.-% and 5 at.-% can be particularly advantageously for some applications.

If the target comprises boron and iron, the sum of boron content and iron content in atomic percentage in the target should not be higher than 20 at.-%. The sum of boron content and iron content should be preferably between 1 at.-% and 5 at.-%.

A method according to the present invention, in which a target comprising aluminium, chromium and boron is used, the boron content helps to reduce or prevent the formation of oxide islands at the surface of targets comprising aluminium and chromium as main components which are used for depositing aluminium chromium oxide comprising coating layers in oxygen reactive atmospheres.

A method according to the present invention, in which a target comprising aluminium, chromium and iron is used, the iron content promotes the formation of (AI,Cr)₂0₃ corundum structures in the aluminium chromium oxide comprising coating layer.

Furthermore the present invention relates to a body coated with a coating comprising at least one aluminium chromium oxide comprising coating layer produced by using a coating method according to one of the embodiments of the present invention described above, in which boron comprising aluminium chromium targets are used for producing aluminium chromium oxide comprising coating layers. The so produced layer should have a boron content in atomic percentage lower than 20 at.-%, preferably between 1 at. -% and 5 at.-% or lower, and consequently, the film morphology of the aluminium chromium oxide comprising coating layer produced in this way will be denser in comparison with a similar produced aluminium chromium oxide comprising coating layer but doesn't containing boron. Similarly, the present invention relates to a body coated with a coating comprising at least one aluminium chromium oxide comprising coating layer produced by using a coating method according to one of the embodiments of the present invention described above, in which iron comprising aluminium chromium targets are used for producing aluminium chromium oxide comprising coating layers. The so produced layer should have an iron content in atomic percentage lower than 20 at.-%, preferably between 1 at.-% and 5 at.-% or lower. The aluminium chromium oxide comprising coating layer produced in this way will exhibit corundum structure or at least largely corundum structure.

Also similarly, the present invention relates to a body coated with a coating comprising at least one aluminium iron oxide comprising coating layer produced by using a coating method according to one of the embodiments of the present invention described above, in which iron comprising aluminium targets are used for producing aluminium iron oxide comprising coating layers. The so produced layer should have an iron content in atomic percentage not higher than 30 at.-%, preferably lower. The aluminium iron oxide comprising coating layer produced in this way will be exhibit corundum structure or at least largely corundum structure. This kind of aluminium iron oxide layers should be deposited in order to have an element composition given by the formula (Al, Fe)₂-_x0₃__y with x < 0.2 and y < 0.2.

Claims

1. Coating method for coating a substrate with at least one aluminium oxide comprising coating layer, the layer being produced by physical vapour deposition from at least one target comprising aluminium as main component in a coating chamber containing oxygen as reactive gas, characterized in that:

• the target further comprises chromium as second main component and boron as minor component or iron as minor component or boron and iron as minor components, or

• the target further comprises iron as minor component.

2. Method according to claim 1, characterized in that the target is made by powder metallurgy.

3. Method according to claims 1 or 2, characterized in that the physical vapour deposition process is a sputtering process, or a magnetron sputtering process, or a high power impulse magnetron sputtering process, or a cathodic arc evaporation process.

4. Method according to claim 1 or 2, characterized in that the substrate temperature during coating process is not higher than 600 °C, preferably not higher than 550 °C.

5. Method according to any of the previous claims 1 to 4, characterized in that during the physical vapour deposition a bias voltage is applied to the substrate that is being coated.

6. Method according to any of the previous claims 1 to 5, characterized in that the oxygen contained in the coating chamber is introduced by using a total oxygen flow of at least 300 seem, preferably of at least 1000 seem.

7. Method according to any of the previous claims 1 to 6, characterized in that the oxygen partial pressure in the coating chamber is about 1.1 Pa.

8. Method according to any of the previous claims 1 to 7, characterized in that the target comprises aluminium and chromium and the atomic ratio Al/Cr in the target is higher than 2, preferably Al/Cr « 70/30, more preferably Al/Cr > 70/30.

9. Method according to any of the previous claims 1 to 7, characterized in that the target comprises aluminium and iron and the atomic ratio Al/Fe in the target is at least 70/30, preferably Al/Fe > 70/30.

Method according to claims 8, characterized in that

- the target comprises boron and the boron content in atomic percentage in the target is not higher than 20 at.-%, preferably the boron content is between 1 at.-% and 5 at.-%, or

- the target comprises iron and the iron content in atomic percentage in the target is not higher than 20 at.-%, preferably the iron content is between 1 at.-% and 5 at.-%, or

- the target comprises boron and iron and the sum of boron content and iron content in atomic percentage in the target is not higher than 20 at.-%, preferably the sum of boron content and iron content is between 1 at.-% and 5 at.-%.

11. Method according to claim 10, characterized in that the boron content in the target reduces or prevents the formation of oxide islands at the target surface during the deposition of the aluminium chromium oxide comprising coating layer.

12. Method according to claim 10 or 11, characterized in that the iron content in the target promotes the formation of (AI,Cr)₂0₃ corundum structures in the aluminium chromium oxide comprising coating layer.

13. Body coated with a coating comprising at least one aluminium chromium oxide comprising coating layer produced by using a coating method according to claim 11 or 12, characterized in that the boron content in the aluminium chromium oxide comprising coating layer in atomic percentage is lower than 20 at.-%, preferably between 1 at.-% and 5 at.-% or lower, and the film morphology of the aluminium chromium oxide comprising coating layer is denser in comparison with a similar produced aluminium chromium oxide comprising coating layer but doesn't containing boron.

14. Body coated with a coating comprising at least one aluminium chromium oxide comprising coating layer produced by using a coating method according to claim 12, characterized in that the iron content in the aluminium chromium oxide comprising coating layer in atomic percentage is lower than 20 at.-%, preferably between 1 at.-% and 5 at.-% or lower, and the aluminium chromium oxide comprising coating layer exhibits corundum structure or at least largely corundum structure.

15. Body coated with a coating comprising at least one aluminium iron oxide comprising coating layer produced by using a coating method according to claim 9, characterized in that the iron content in the aluminium iron oxide comprising coating layer in atomic percentage is not higher than 30 at.- %, preferably lower, and the aluminium iron oxide comprising coating layer exhibits corundum structure or at least largely corundum structure/ and has preferably an element composition given by the formula (Al, Fe)2_-X0₃._y with x < 0.2 and y < 0.2.